1. Field of the Invention
The present invention relates to a base structure of an end area of a word line of a semiconductor memory device.
2. Description of the Related Art
In recent years, in a semiconductor memory device such as a NAND type nonvolatile semiconductor memory device, a space between adjacent word lines (control gate lines) having a line & space structure has become narrow in accordance with the increase in capacity. For this reason, the space between the word lines is not completely filled with insulating materials, so that a void is easily formed in the space between the word lines. In some cases, the void extends to an end area of the word line along the word line.
The end area of the word line has a structure in which the distance between the word lines broadens in order to secure an area for providing each contact hole to the word lines.
For instance, a tip of each word line shifts little by little in an extending direction of the word line, and a pattern of the end area of the word line, as a whole, results in a single-edged shape or a double-edged shape. In addition, a fringe is formed at the end area of the word line.
For this reason, in some cases, an opening is formed on the void in the end area of the word line.
On the other hand, conventionally, in the NAND type or NOR type nonvolatile memory device, low resistance of the word line is attained by constituting an upper portion of the word line with tungsten silicide (WSi), however, in recent years, for further low resistance, for instance, employment of the silicide structure by using cobalt is started (for instance, refer to Jpn. Pat. Appln. KOKAI Publication No. 2007-73887).
Formation of the silicide structure by using cobalt uses a process in which, unlike the conventional formation of tungsten silicide, cobalt metal is deposited on a conductive polysilicon film, silicide is formed by reacting the cobalt metal with the polysilicon film by heat treatment, and unreacted cobalt metal is removed by wet etching.
Here, when the opening is formed in the void, dissolved liquid of metal silicide generated at the time of wet etching flows into the void via the opening, and remains in the void.
The dissolved liquid of the remaining metal silicide in the void becomes a metal ion and moves in an insulating film when high voltage is applied between two word lines which sandwich the dissolved liquid therebetween, thereby causing a short-circuit between these two word lines.
Therefore, in order to improve the reliability of the semiconductor memory device, a new technique for preventing such phenomenon must be developed.